During vehicle travel there are many instances where the vehicle must stop and idle. For example, vehicles stop and idle at traffic signals, cross-walks, stop signs and the like. Vehicles including an automatic transmission can experience a vehicle roll-back event during periods of idle. More specifically, if a vehicle is idling on a surface that has a sufficient grade, the vehicle may be induced to roll backward.
The automatic transmission is driven by the engine through a torque converter. During idle, the torque transferred to the transmission is generally sufficient to enable slight forward rolling of the vehicle (i.e., creeping) when on a flat surface. When on a slight grade, the torque transferred to the transmission is generally sufficient to hold the vehicle at a stand still and prevent vehicle roll-back. On higher grades, however, vehicle roll-back can occur, which causes reverse torque transfer through the transmission.
Conventional methods of detecting and inhibiting vehicle roll-back include automatic control of a vehicle brake system, increased engine torque output and a mechanical brake within the transmission. These methods, however, are more complex and costly than desired.